On Fri, Jun 19, 2015 at 09:57:22AM +0200, Paolo Bonzini wrote: > > > On 19/06/2015 09:56, Michael S. Tsirkin wrote: > > On Thu, Jun 18, 2015 at 06:02:46PM +0200, Paolo Bonzini wrote: > >> > >> > >> On 18/06/2015 16:47, Michael S. Tsirkin wrote: > >>>> However, with Igor's patches a memory_region_del_subregion will cause a > >>>> mmap(MAP_NORESERVE), which _does_ have the effect of making the hva go away. > >>>> > >>>> I guess one way to do it would be to alias the same page in two places, > >>>> one for use by vhost and one for use by everything else. However, the > >>>> kernel does not provide the means to do this kind of aliasing for > >>>> anonymous mmaps. > >>> > >>> Basically pages go away on munmap, so won't simple > >>> lock > >>> munmap > >>> mmap(MAP_NORESERVE) > >>> unlock > >>> do the trick? > >> > >> Not sure I follow. Here we have this: > >> > >> VCPU 1 VCPU 2 I/O worker > >> ---------------------------------------------------------------------------------------- > >> take big QEMU lock > >> p = address_space_map(hva, len) > >> pass I/O request to worker thread > >> read(fd, p, len) > >> release big QEMU lock > >> > >> memory_region_del_subregion > >> mmap(MAP_NORESERVE) > >> > >> read returns EFAULT > > > > Why doesn't it EFAULT without mmap(MAP_NORESERVE)? > > Doesn't memory_region_del_subregion free the memory? > > No, only destruction of the memory region frees it. address_space_map > takes a reference to the memory region and address_space_unmap releases it. > > Paolo Confused. So can we call mmap(MAP_NORESERVE) in address_space_unmap after we detect refcount is 0? > >> wake up VCPU 1 > >> take big QEMU lock > >> EFAULT? What's that? > >> > >> In another scenario you are less lucky: the memory accesses > >> between address_space_map/unmap aren't done in the kernel and > >> you get a plain old SIGSEGV. > >> > >> This is not something that you can fix with a lock. The very > >> purpose of the map/unmap API is to do stuff asynchronously while > >> the lock is released. > >> > >> Thanks, > >> > >> Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
